Traveling-wave tubes



Qct. 8, 1957V R. ADLER TRAvELING-WAVE TUBES n Filed Nov. 27, 1953 2 sheets-sheet 1 vC. 8, 1957 R, ADLER 2,809,320

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aired States f arent O aan TRAVELING-WAVE TUBES Robert Adier, Northiieid, ill., assigner to Zenith Radio Corporation, a corporation of liiinois Application November 27, 1953, Serial No. 394,798

7 Ciaims. (Cl. S15-3.5)

This invention relates to new and improved electroni discharge devices of the traveling-wave type. More particularly, the invention is directed to traveling-wave tubes suitable for use as ampliiiers operable over a relatively wide range of frequencies.

In the recent past, the Federal Communications Comvision receivers have found it necessary to provide termii nal equipment adapted to receive programs transmitted within this frequency range. One of the most diicult problems presented in the construction of a television receiver of this type results from the fact that conventional intensity-control electron tubes (triodes, pentodes, etc.) are not well suited for use as amplifiers within the ultra-high-frequency range; more particularly, it is extremely diflicult to achieve uniform gain throughout the U. H. F. range with tubes having practical dimensions and tolerances. Accordingly, it has generally been considered preferable to apply the received signal directly to a heterodyning stage without amplification. In this event, however, the picture reproduced by the receiver is often seriously disturbed by thermal noise.

One type of electron-discharge device which is capable of providing amplilication over a relatively wide range of high frequencies is the conventional traveling-wave tube. in these tubes, a radio-frequency signal is applied to a low-velocity wave-transmission line, which, in its simplest form, may comprise a helically wound conductor. An electron stream is directed along a path closely adjacent to the helical line; usually, the electron beam path coincides with the axis of the helix. The velocity of the electrons in the beam is made substantially equal to the eiective velocity of the radio-frequency signal wave traveling along the line. The electron beam is velocity-moduated by the electrostatic field developed by the signal wave traveling along the line, and, in turn, induces current in the line which may amplify the radio-frequency signal. However, such traveling-wave tubes are much too large and expensive for use in a television receiver.

A little-known variant of the traveling-wave tube comprises a device adapted for push-pull or transverse mode operation, as opposed to the longitudinal or velocitymodulation mode of operation employed in the conventional tubes. A transverse-mode traveling-wave tube may comprise a pair of low-velocity wave-transmission lines mounted in substantially parallel spaced relationship with respect to each other. A radio-frequency input signal is applied in push-pull relationship to the two wave-transmission lines, and an electron stream is projected along a path intermediate the two transmission lines at a velocity substantially equal to the eective propagation velocity of the signal wave along the length of the lines. Consequently, each electron of the stream is subjected to a transverse electricield which travels along the beam path ice at approximately the same velocity, and is deected from its original path toward one of the lines, depending upon the polarity of the field to which it is subjected. The resulting transverse motion of the electron stream toward one helix and away from the other helix spreads the stream into a wave-like pattern and this pattern, moving along the stationary lines, induces currents in the lines which, in a rather complex manner, may tend to reinforce the original radio-frequency signal so that exponential amplication is attained.

In both conventional and transverse-mode travelingwave tubes, it is essential that the electron stream be coniined to a relatively narrow path so that the electrons are not collected by the wave-transmission lines. A magnetic ield extending throughout the length of the electron beam path is generally employed to contne the electrons to that path and to prevent dispersion of the beam. A relatively bulky and expensive electromagnetic coil surrounding the entire traveling-wave tube is usually utilized for this purpose. However, such a structure is not desirable in apparatus such as a television receiver, where space and cost considerations are of paramount importance. 1

lt is a primary object of the invention, therefore, to provide a new and improved electron-discharge device suitable for use as an amplifier over a relatively wide range of ultra-high frequencies.

it is a further object of the invention to provide an electron-discharge device, capable of operating as a broad band U. H. F. amplifier, which is relatively small in size but which provides an acceptable degree of amplication.

lt is a speciiic object of the invention to provide a new and improved electron-discharge device of the transversemode traveling-wave type in which the electron stream is eectively confined to a predetermined path without requiring the use of a magnetic collimating system.

It is another object of the invention to provide a new and improved transverse-mode traveling-wave tube in which thermal noise is substantially minimized.

It is a corollary object of the invention to provide a traveling-wave tube which is relatively simple and expedient to construct and economical to manufacture.

An electron-discharge device of the traveling Wave type, constructed in accordance with the invention, comprises an electron gun for projecting a beam of electrons along a reference path. A plurality of electrodes consecutively disposed along a predetermined portion of the reference path and individually comprising first and second juxtaposed mutually insulated conductive elements respectively on opposite sides of the reference path are provided for inducing transverse undulations of the beam. Additional electrodes are disposed along the reference path inter-v mediate successive pairs of the first-mentioned electrodes and are adapted to be maintained at a diierent unidirectional operating potential than that applied to the irstmentioned electrodes to constitute therewith a series of electrostatic lenses along the predetermined portion of the reference path for confining the resulting transverse-undulating beam to the region of the reference path. Inductance means interconnect at least some of the electrodes to form a wave-transmission line. Further means, coupling the wave-transmission line to an input signal source, are provided for providing transverse electric signal elds between the juxtaposed conductive elements and for producing a signal Wave which travels along the line and interacts with the transverse-undulating beam to provide substantial reinforcement of the traveling signal wave. The wave-transmission line has a substantially uniform low wave propagation velocity in a direction parallel to the reference path and an electrical length which is largev f 3 relative to the effective wave length of the traveling signal Wave.

The features of the invention which are believed to be novel are set forthwith particularity in the appended claims. kThe organization and manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings,rin which like reference numerals refer to like elements in the several gures, and in wlnch:

Figure l is a cross-sectionalview, partially schematic, of one embodiment of an electron-discharge device constructed in accordance with the invention, and includes a schematic representation of a simplilied amplifier circuit;

Y `Figure la is an explanatory diagram illustrating certain operational features of the apparatus of Figure l;

Figure 2 is a cross-sectional view taken along line 2 2 in Figure 1;

yFigure 3 represents a second embodiment of the invention, as seen in cross-section, a portion of the structure being' illustrated schematically;

Figure 4 is a cross-sectional view taken along line 4-4 in Figure 3;

Figure 5 shows another embodiment of the invention with a portion of the device and its associated circuitry illustrated schematically; and

Figure 6 is a cross-sectional view of the traveling-wave tube of Figure 5, taken along line 6-6 therein.

Theembodiment of the invention shown in Figure l comprises an electron-discharge device or traveling-wave tube 2l); tube 20 includes a cathode 10 having an electronemissive surface 11. A focusing electrode 12 is mounted in parallel spaced relation to surface 11 and includes a Centrallylocated slot 13. .An accelerator electrode 14 including an aperture 16 is included in device 20 and is positioned adjacent focusing electrode 12 with slot 16 opposite slot 13. A beam-limiting electrode is mounted in spaced relationship to accelerator 14 on the side of the accelerator opposite electrode 12; electrode 15 includes a Ybeam-limiting aperture 17 aligned with aperture 16. Cathodel() and electrodes 12, 14 and 15Vcomprise an electron gun 19'for projecting an electron beam along a reference path indicated vby a dash line A comprisingl the center plane of therpath'; reference path A terminates at a'collector electrode 18 positioned at the opposite end of tube 20. Y

Preferably, the electron' beam developed by gun 19 is sheet-like in form; in other words', the electron beam has Yone principal cross-sectional dimension which is very much greater than a second principal cross-sectional dimension, these dimensions being generally determined by the conliguration of slot 17.V For the illustrated structure, the lengths of electrodes 10, 12, 14, 15 and 18, taken in a direction perpendicular to the plane of the drawing of Figure l, are very much greater than the widths of slots 13, 16 and 17, so that the electron beam is of rectangular cross-sectional configuration and has a thickness very much smaller than its width. It should be noted that the invention may be advantageously employed in Vstructuresiwhich'do not utilize a ribbon or sheet beam; however, a beam of this type is normally quite advantageous in a weak-signal amplifier, due to the fact that it permits realization'of increased amplication from a tube of given maximum dimensions.

Y A lirst wave-transrnission line 21 is disposed adjacent reference path A intermediate electrode 15 and collector 18. Transmission line 21 comprises a plurality of conductive lens elements or plates 40 which are disposed along a predetermined portion Z of reference path AL Line 21 further includes rst'and second'helical conductors 22 and 23 wound in bilar fashion about a longitudinal support member 24. The individual Yturns 'of winding 22 are cross-hatched in the drawing, whereas' the turns of Winding 23, although also shown in cross section, are unshaded in order to differentiate the two windings from one another. lt should be understood that the size of windings 22 and 23 and conductive plates 40 has been greatly exaggerated in Figure l and inrsucceeding figures in order to facilitate the presentation of the inventive concept, and that only a relatively few turns of each winding are illustrated as compared to the number which may be employed in practice.

Winding 22 is electrically connected to that one of plates 4i) most closely adjacent to electron gun 19, designated as plate dna, and to succeeding alternate ones of the conductive plates. Selected'turns of winding 23, on the other hand, are electrically connected to each of the remaining lens plates 411. Windings 22 and 23 are electrically ntercoupled for radio frequencies at the end of line ..1 adjacent gun 19 by means of a capacitor 28; capacitor 2S is shown mounted within the envelope 29 of tube 24.1. lf preferred, the electrical coupling between the two windings may be made externally to envelope 29, although it is usually desirable to eiect this coupling within the envelope in order to minimize the number of external leads. At the other end of line 21, adjacent collector 18, windings 22 and 23 are again electrically inter-coupled by means of a capacitor 30. Capacitors 28 and 30 constitute effective short-circuits at the inputsignal frequency while isolating the bilar windings from each other with respect to D. C. or average potential.

Tube 2S also includes a second low-velocity wavetransmission line which is disposed adjacent reference path A on the opposite side of the path from line 21. Line 25 is essentially identical, electrically and physically, with line 21, and comprises a plurality of conductive plates 41 alternately connected to selected turns of a pair of bilar conductive windings 26 and 27. Winding 26 is cross-hatched and helix 27 is unshaded so that the individual coil elements of the two windings may be readily distinguished. Two capacitors 31 and 32 are provided to couple windings 26 and 27 to each other for radio fre quencies only.

lt will be observed that conductive plates 41 are dis-A posed immediately opposite plates 4b of line 21. Moreover, the conductive plates are preferably equally spaced from the center plane of path A and deiine the maximum useful width d of the beam path. The length Z of the wave-transmission lines determines the portion of path A in which interaction between the electron beam and a signal wave traveling along lines 21 and 25 may occur.

Tube 20 may be provided with a suitable base for envelope 23 and a separate heater lament may be included in cathode 1b; because these structural details are familiar in the art, they are not illustrated in the drawings. Envelope 29 may Ybe of conventional receiver-tube size.

. After wave-transmission lines 21 and 25, collector 18,

and the electrodes comprising electron gun 19 have been mounted within envelope 29, the envelope is evacuated and gettered in any manner known in the art. Y

A simplilied amplilier circuit Vfor tube 26 has been schematically illustrated in Figure 1 in order to facilitate the description Yof the operation of the tube. A balanced signal source 33 is connected between winding 23 of line 21 and winding 27 of line 25. Of course, signal source Y 331s lalso electrically coupled to Vwindings 22 and 26 through capacitors 2S and 31 respectively; however, there is no direct conductive connection between the signal u source andthe latter'two windings. Source 33 may comprise any suitablersource of radio-frequency signals, and may, for instance, constitute the termination of a balanced television antenna. A balancedload circuit 34 is connected between winding 22 and 26 of transmission llinesv21 and 25 respectively and isY coupled to` windings 23 and A27 through capacitorsstl and 32. Y

Cathode 10 is iconnected to a plane of reference potential, here'illustrated as ground, and focusing electrode 12 may also be connected to ground. If preferred, electrode 12 may be connected toa suitable source of automatic gain control potential (not shown) to maintain a relatively constant output signal amplitude. Accelerator 14 is connected to a rst source of positive unidirectional operating potential B1-|-, and electrode 15 is connected to a second positive voltage source B2}-. A third source of positive unidirectional potential, B34-, is connected to signal source 33 to provide a preselected constant average potential for winding 23 of transmission line 21 and winding 27 of line 25. Similarly, a fourth D. C. voltage source Bt-lis connected to load circuit 34 to apply a positive D. C. voltage to transmission-line windings 22 and 26. Collector 18 is electrically connected to an additional source of positive operating potential, B5}-. It will be understood, of course, that all of the B-ivoltage sources may comprise separate taps on a single unidirectional or D. C. voltage source. Furthermore, some of the B-- potentials (e. g., Bi-land B5|-) may be of the same value.

In traveling-wave tubes constructed in accordance with known techniques, common experience indicates that the use of electrodes which intercept any appreciable portion of the electron beam may produce highly undesirable partition-noise effects; accordingly, the electron guns of conventional tubes employ electrode structures in which an attempt is made to avoid interception, by the gun electrodes, of any substantial portion of the beam. One percent interception, for instance, is considered excessive. However, it has been found, surprisingly enough, that these partition-noise effects are not significant in transverse-mode traveling-wave tubes; indeed, electron guns including electrodes which intercept fifty percent or more of the total beam current have been found to provide inherently better noise characteristics, in transverse-mode tubes, than electron guns of the type utilized in conventional tubes. Consequently, electrode is preferably constructed to intercept a substantial potrion of the total beam current, preferably greater than fifty percent.

As seen in cross-sectional view Figure 2, the thickness of the electron beam, which is determined by dimension t of slot 17, is very much smaller than its height, which corresponds to slot dimension h, so that the beam has a cross-sectional coniignration corresponding to an elongated rectangle. It should be understood that although a rectangular cross-sectional configuration has been illustrated, other beam configurations may be employed. Support member 24, upon which windings 22 and 23 of line 21 are wound, is essentially I-shaped in cross-sectional conguration, and the longitudinal support member 35 upon which windings and 27 are mounted is similarly shaped.

Referring again to Figure 1, when device 20 is placed in operation a radio-frequency signal is applied to wavetransmission lines 21 and 25, in push-pull relationship, from source 33. Due to the helical configuration of the windings which form the wave-transmission lines, each of lines 21 and 25 has a wave-propagation velocity in a direction parallel to reference path A which is considerably smaller than the propagation velocity of electromagnetic radiation in free space. The actual effective wavepropagation velocity of the lines is a matter of design choice and may be as low as one one-hundredth (0.01) of the free-space propagation velocity. Electrons emitted from surface 11 of cathode 10 are focused by passing through slot 13 of electrode 12 and are accelerated as they traverse slot 16 of accelerator 14, due to the positive operating potential applied to the accelerator from source 131+. Part of the electron stream then passes through aperture 17 of electrode 15, which may be held at a potential somewhat below that of accelerator 14 but positive with respect to the cathode. Preferably, the average potentials supplied to the windings of lines 21 and 25 should be different from that applied to electrode 15 from source Bz-{, so that an electrostatic focusing lens is formed be` tween electrode 15 and the adjacent edges of wavetansmission lines 21 and 25. The electrons of the beam continue along reference path A and are collected by electrode 18. The beam velocity is determined by the average of the D. C. potentials of transmission lines 21 and 25 with respect to cathode 10.

In order to reach a more complete understanding of the advantages and useful qualities of the invention, the amplitier illustrated in Figures l and 2 may rst be considered as operating without the benet of any collimating or focusing iield tending to confine the electron beam to maximum width d of reference path A. The velocity of the electron beam along path A may then be adjusted so that it is approximately equal to the wave-propagation velocity of the signal wave traveling along transmission lines 21 and 25. Each electron or group of electrons instantaneously emerging into the portion of reference path A defined by transmission line length Z is subjected to a transverse electric iield established by the application of radio-frequency signals from source 33, in phase opposition, to the two transmission lines. Because the electron and wave-propagation velocities are equal, each individual electron is continuously deflected in a given direction and begins to move transversely with respect to path A as well as parallel thereto. As the electrons move away from the center plane of the reference path, they form a wave pattern which, by moving along the wave-transmission lines, induces a signal current in the lines; this induced current is out of phase with respect to the original eld supplied by source 33.

Interaction between the traveling-wave field and the electron stream leads to the emergence of three separate waves in place of the original signal wave; only one of the three waves grows exponentially as it travels along the transmission lines. This process has been'analyzed in chapter 13 of the book entitled Traveling-Wave Tubes, by I. R. Pierce, published by D. Van Nostrand Co., Inc., New York, 1950.

Operation of the amplifier of Figures l and 2 under the conditions just described might be highly successful if the electrons projected from emissive surface 11 of `cathode itl all followed paths exactly parallelvto reference path A and if space-charge etfects could be ignored. In practice, however, this is not possible. On the average, each of the electrons emerging from electron gun 19 has some initial transverse velocity which causes the beam to disperse and, in addition, spacecharge effects tend further to spread the beam. Consequently, if no provisions are made for focusing or collimating the electron beam as it traverses reference path length Z, it is extremely difiicult to secure any appreciable `gain from traveling-wave tube 20, since the electron beam rapidly disperses and is collected by the wave-transmission lines. Conventionally, magnetic collimating elds have been employed to restrict the transverse excursions of the beam electrons; the structures employed to develop the magnetic collimating field, however, are considered excessively bulky and expensive for domestic television receivers and similar applications.

Traveling-wave tube 20, on the other hand, includes an electrostatic focusing structure for confining the electron beam within maximum width d of reference path A; width d is limited by and must be smaller than the spacing between lens elements 4i) and 41. The bifilar construction of transmission lines 21 and 25 and the capacitive coupling utilized between the individual helical windings at each en'd of the line makes it possible to establish the individual windings of the wave-transmission lines at different operating or D. C. potentials. Thus, winding 23 of wave-transmission line 21 may be held at an average potential which is positive with respect to winding 22; similarly, winding 27 of line 25 may be made positive with respect to winding 26. The difference in operating potential between the wave-transmission line windings results in the establishment of a similar difference in average potential between adjacent ones 0f lens elements 4@ and 41, due to the fact that alternate ones" of plates 40 are connected to transmission line windings 22 .and 23, whereas the corresponding plates 41Y are alternately connectedrto windings 26 and 27. This condition is illustrated in Figure'la, which comprises a schematic representation of the lens elements; the relativeV sizes and spacings of the elements illustrated therein have been distorted somewhat intorder to assist in explaining the ligure and to provide more space. The radiofrequency potential resulting from the signal wave traveling' along lines. Zit'and 25 is relatively small in comparison with the difference in D. C. potential between theY individual windings of each line.

Asshown in Figure la, an electron entering the portion of reference path A bounded by the wave-transmission lines may have a velocity component in the transverse direction indicated by arrows y as wellas a principal velocity component parallel'to referenceV path A. The transverse velocity oi the electron beam may result from thermal or space-chargeellects or other factors. The electron may enter the interaction space bounded by lensV elements itl and el along a hypothetical path A', and, at the outset, is subjected to an electrostatic field primarily determined by the average or steadystate potential of lens elements lila and la which, in turn, is determined by the D. C. voltage applied to transmission Vline windings 23 and 27 from source Bs-j- (Figure'l). As the electron continues along path A', it reaches thespace between plates 49a, lila and the next pair or" lens elements designated l-ilb, alb. Because plates 40a and lila are connected to transmission line windings 22 and 26 and are maintained at a considerably higher potential than lens elements 4Gb and alb, the electron encounters a convergent electron lens action which tends to deliect the electron toward center plane A of the reference path. The electron continues along path A', and, upon reaching the space bounded by plates 4%, 41]; and dile, lo, enters another convergent electrostatic lens. Consequently, the electron is again ydeflected toward center plane A. Thus, as the electron proceeds along path A it is subjected to a periodic electrostatic lens iield effectively constituting a series of convergent electrostatic lenses. The periodic lens field tends to deflect the Velectron toward reference path center plane A with a force which-is proportional to the displacement of the electron from that plane; accordingly', the lens iield is generally equivalent to a transverse elastic field and conhnes the electrons of the beam to maximum path width d.

The focal length of each of the electrostatic lenses formed between adjacent pairs of lens elements is proportional to the transverse spacing between those elements and is a function of the ratio between the D. C. potentials of the lens elements with respect to cathode 16. The focal lengths of the electrostatic lenses, in conjunction with the spacing s between adjacent lenses, determine the distance along path A which is traversed by an electron having a given initial transverse velocity before that electron crosses the center plane of the reference path. Consequently, as shown by trajectory A', each electron (other than those having no initial transverse velocity) follows a substantially sinusoidal path which is symmetrical with respect to reference-path center plane'A. A second hypothetical electron path A illustrates the trajectory of an electron having a diilerent initial transverse velocity and a different starting position from the electron following path A. As indicated by the trajectory A", the magnitude and direction of the initial transverse velocity and the original displacement of the electron with respect to center plane A do not affect the vwavelength Le of the sinnsodal paths followed by the individual' electrons.

For a given structure, wavelength Le is determined by the'strength or" the lens iield produced by the D. C. potential difference between the individual windings, and by the average of their individual D. C. potentials which establishes theaverage velocity of the electron stream.'Y

As'each electron follows its individual trajectory, it 'carries out a transverse harmonic motion at a frequency we equal to its average velocity divided by wavelength Le. This transverse motion is analogous to the motion of a mechanical resonator such as a vibrating reed; a periodic force having a 'frequency equal to the natural frequency of such a resonator produces a periodic motion of linearly increasing amplitude. Consequently, the electron stream may be said to exhibit a transverse resonance at Vthe frequency we. Y

The frequency of the signal applied from source 33 may be designated wD and the propagation velocity of the undisturbed signal wave traveling along the Ilines may be taken as vo. The average velocity of the electrons may be designated ve. l

By proper choice of the D. C. potentials apphed to the wave-transmission lines, the velocity of the electron beam may be adjusted so that EL 1) v.-w.(1+wo the current induced in wave-transmission lines 21 andv 25 by the pattern of transversely vibrating electrons moving along path A is in phase with the signal wave applied from source 33, so that gain is achieved. The amplitude of the signal wave, at any point along the lines, may be expressed as (2) 15:1/2Eo (estar-az) En is the initial amplitude of the signal applied by sourceV 33, e is the natural logarithmic base, z is the distance along the Wave-transmission lines between the input end of the lines and the point of measurement, and a is the growth constant of the signal wave. In the conventional longitudinal-mode traveling-wave tube, the growth constant is determined by a cubic expression, Whereas in tube Ztl growthV constant a may be represented by a quadratic equation. This indicates that an exponentially attenuated wave as well as an exponentially growing wave should exist along wave-transmission lines 2l and 25; this has been determined to be true. The fact that the initially applied signal is efectively divided into these two wavesV accounts for the factor 1/2 in the above equation. For a tube which is long enough to afford substantial gain, the attenuated wave is of negligible effect as compared to the amplified wave, so that a simplified expression for the gain of such a long tube is This compares quite favorably with the conventional velocity-modulation traveling-wave tube, in which the cubic expression for the growth constant indicates division of the applied signal into three waves, so that the equation for a long tube includes a factor of only 1/3 instead of 1/2.

lt should be noted that the phase conditions within tube 2!) which result in the direct addition of the induced current to the original signal current in wave-transmission lines 21 and 25 prevail accurately only so long as the amplitude of the driving or signal field does not change along the'length of the wave-transmission lines. Actually, the driving eld increases'l continuously, so that a phase error occurs. This phase error, however, is lrelatively'small and may be readily eliminated by minor adjustments in either the transverse resonance frequency:

or the electron beam velocity; such adjustments may conveniently be made by varying the applied potentials from one or more of the D. C. sources B1+ to 134+ inclusive.

In order to achieve effective operation of tube 20, several conditions should be met. To avoid difficulties presented by lens aberrations, the spacing s between adjacent lenses (Figures l and la) should be greater than three times the maximum permissible Width d of reference path A (the proportions illustrated in Figures l and la are at variance with this condition to avoid overcrowding). The effective wavelength Le of the lens iield (Figure la) should be equal to or greater than three times lens spacing s. Furthermore, transmission-line length Z must be large in relation to the effective wavelength of the signal wave as it travels along the wa. transmission lines; this latter condition must be met in order to achieve appreciable gain. While the lumped reactance structure of the wave-transmission lines inherently forms a low-pass filter, the cut-olf frequency of the resulting lter may be established well above the desired signal frequency by a judicious choice of dimensions in accordance with known design techniques. All of the dimensional and operational characteristics set forth in this paragraph apply to the embodiments of the invention illustrated in Figures 3-6 as well as to the device described in connection with Figures 1 2.

Th electrostatic lenses established along path A determine the transverse resonant frequency of the electron beam and, at the same time, conne the beam within width d so that it does not irnpinge upon the wave-transmission lines. Consequently, it is possible to make lines 21 and 25 suiciently long to achieve useful gain from tube 2%. The electrostatic lens field centers the beam about center plane A, as contrasted to the mere collimating action of a conventional magnetic field, so that width d may be held to a minimum. This facilitates close couphng between the electron beam and the wave-transmission lines and permits the realization of greater amplication in a tube of given overall size.

It has been determined that the exponential gain or growth constant a of a transverse-mode traveling-wave tube such as device 29 is adversely affected by stray capacities along the wave-transmission lines. In order to minimize such stray capacities, support members 24 and 35 preferably have a transverse cross-sectional area which is as small as possible; in addition, the material from which the supports are made should have a low dielectric constant. The I-shaped cross sectional configuration for the support members illustrated in Figure 2 is advantageous in this respect in that it presents a relatively small cross-sectional area which is largely separated from the individual turns. Support members 24 and 35 may be formed from ceramic, glass, or other dielectric materials adapted for use in a vacuum. Helix windings 22, 23, 26 and 27, of course, may be formed from copper, molybdenum, or other conductive material suitable for use in a vacuum.

In the embodiment of the invention illustrated in Figures l and 2, the electrostatic lens eld employed to confine the electron beam within path Width d is established by the potential diiferences between individual lens elements. In this embodiment, and in all modifications to be described hereinafter, the field created by the signal wave traveling along transmission lines 21 and is electrostatically coupled to the electron beam by means of a series of separately identiiiable conductive plates; thisjstructure permits separate fabrication of the capacitive and inductive elements of the wave-transmission lines. However, it is sometimes advantageous to construct the transmission line-lens structure so that the windings themselves are directly coupled to the beam; structures of this general type are described in the copending application of Robert Adler, Serial No. 394,797, entitled Traveling Wave'Ampliiiers, filed concurrently `10 herewith and assigned to the saine assignee as the presi ent invention. The periodic electrostatic lens structure may also be put to advantageous use in traveling-wave tubes employing a single Wave-transmission line and specically adapted for longitudinal-mode operation.

Figure 3 illustrates a second embodiment of the invention which in many respects in similar to that of Figures l and 2. The embodiment of Figure 3 comprises an electron-discharge device 50 including an envelope 29 and an electron gun 19 mounted at one end of the envelope with an anode or collector 18 mounted at the opposite end of the envelope. The construction of these elements may be the same as the corresponding portions of the device shown in Figure l. Electron gun 19 projects a stream of electrons which generally follows a reference path A and which is collected at anode 18.

Tube 56 further comprises a first low-velocity wavetransrnission line 51 which is disposed adjacent to one side of reference path A and is interposed between electron gun 19 and anode 18. Wave-transmission line 51 comprises a helical conductive winding 52 supported by an insulating member 53. A series of conductive lens elements or plates 54 are interposed between helical winding S2 and reference path A and are individually connected to spaced turns along the transmission line winding. A second low-velocity Wave-transmission line 5S is disposed along path A opposite line 51; line 55 includes a helical conductive winding 56, supported by an insulating member 57, and a group of conductive plates 58 connected to spaced turns of winding 56 and located directly opposite plates 54 of line 51. A plurality of apertured lens electrodes 59 are individually disposed between adjacent pairs of plates 54 and 58; lens electrodes 59 are electrically connected to each other and to a source of operating potential B4-}-. The ends of conductive windings 52 and 56 adjacent electron gun 19 are electrically connected to signal source 33 and to a source of unidirectional positive potential B3-{-, whereas the other ends of the helical windings are each electrically coupled to load circuit 34;. The electrical connections for anode 18 and for the individual elements of electron gun 19 may be the same as previously described in connection with the apparatus of Figure l. Support members 53 and 57 may be formed from ceramic or other suitable insulating material, whereas lens electrode plates 54 and 58 and lens elements 59 may be fabricated from any suitable conductive material.

The cross sectional View of Figure 4 provides a further illustration of the alignment and construction of the various elements of tube 50. In this preferred construction, the support members are essentially I-shaped in cross section so that stray capacities along the line are minimized. Moreover, and as in the embodiment of Figures 1 2, a sheet or ribbon beam of rectangular crosssectional area is employed.

When the apparatus of Figure 3 is placed in operation, the electron stream developed at cathode surface 11 is accelerated and focused by Ipassage through the apertures of electrodes 12, 14 and 15 and projected along path A toward anode 1S. As the electrons of the beam traverse that portion of reference path A bounded by lens plates 54a and 58a, they pass through a region, essentially free from steady-state electrostatic fields at the D. C. potential applied to windings 52 and S from source B11-I. As the electrons enter the space between plates 54a, 58a and Sdib, S817; however, they are subjected to a lens field, due to the fact that the D. C. or average potential applied to lens electrode 59a is made substantially different from that applied to the helical windings. Electrodes 59 may be maintained at a potential either above or below the D, C. potential of plates 54 and 53; preferably, lens electrodes 59 should be at the higher potential. The uni-v potential lens formed by plates 54a, 58a, 54h, and 5811 and electrode 59a directs the electrons toward the center plane of reference path A. As the electrons continue "11 along the Ipath, they are again subjected to a convergent electrostatic lens field as they enter the portion of path A intermediate yplates Sb, 58h and plates 54e, 58C. Thus, the electron stream passes through a series of electrostatic lenses as it traverses the length of reference path A, the strength of each of the lenses being determined by the D, C. potentials applied to the heiical windings from source Bij and to lens electrodes 59 from source B4-j-. It will be immediately apparent that under these conditions transmission lines 51 and 55 function in a manner analogous to lines 21 and 25 of Figure l and cooperate with the electron beam projected along path A to exponentially amplify the signal supplied from source 33. As in the embodiment of Figures l and 2, the electron beam is confined to its path bythe electrostatic lenses which, in part, are formed by a series of conductive plates forming a part of the wave-transmission lines.

Figure 5 is a partially schematic, partially cross-sectional view of an electron-discharge device 70 representing a further embodiment of the invention. Tube 7@ is in many respects similar to the devices described inFigures 1-4 and includes an envelope 2.9, an electron gun 19 mounted at one end of the envelope, and a collector electrode 16 mounted at the opposite end of the envelope. As before, electron gun 19 generates and projects a beam of electrons along reference path A toward collector 18.

Tube 7i) further includes a iirst low-velocity wavetransmission line 71 comprising an insulating support member 72 disposed adjacent reference path A, a group of conductive lens elements 73 mounted in spaced relation along support member 72 on the side thereof adjacent reference path A, and a plurality of inductance coils 74 individually connected in series between adjacent ones of conductive elements 73. Tube 7i) also includes a second low-velocity wave-transmission line 75 which is substantially identical to line '71 and is symmetrically disposed on the opposite side of reference path A. Line 75 includes a support member 76, a plurality of spaced conductive plates 77 positioned adjacent reference path A, and a plurality of coils 78 serially interconnecting the conductive plates. A series of apertured lens electrodes 79 are disposed along path A intermediate transmission lines 71 and 75 at positions corresponding to the locations of helical coils 74 and 78. Lens elements 73 and 77 and electrodes 79 may comprise conductive plates or conductive deposits on supports 72 and 76.

The signal-transmission path of line 71 is delined by the series-connected lens electrodes 73 and coils 74, whereas the wave-transmission path of line 75 is established by the series-connected plates 77 and coils 78. The .f

ends of the two wave-transmission lines adjacent electron gun 19 are connected to signal source 33 and to a source of operating potential B34-, Whereas the ends of the lines adjacent collector 18 are electrically coupled to load circuit 34. Apertured lens elements 79 are electrically connected to each other and to a source of operating potential B4-{. The electrical connections for collector V18 and for the individual elements of electron gun 19 may be the same as in the previously described embodiments.

Figure 6 provides a cross-sectional view of tube 7) taken Valong line 6 6 in Figure 5.. As indicated in Figure 6, support members 72 may comprise relatively thin sheets of mica or other suitable insulating material. Lens electrodes 73 and 77 may be fabricated from any conductive material for use in a vacuum, and coils 74 and 7S are simple electrical inductance coils. Lens electrodes 79 may be electrically interconnected and supported by means of a metal rod or strap 80.

in operation, tube 70 is generally similar to the embodiments in Figures 1 4. The steady-state operating potential applied to transmission lines 71 Vand 75 from source Ba-lis established at a substantially diierent level from the operating'potential of lens electrodes 79 as determined by source 34+. Accordingly, as the electron beam enters that portion of path A intermediate plates Y 73a and77a on the one hand and plates 73b and 77b on the other hand, it is subjected to a convergent electrostatic lens field which centers the beam upon reference path A. the space between plates 73h, '77b and plates 73e, 77C, it is again subjected to an electrostatic lens field and is converged upon the center line of the path. Thus, as in the previously described embodiments, the electron beam is subjected to a periodic lens lield which contines the beam .within maximum permissible width d and at the saine time establishes a transverse resonance frequency for the beam.

Each of the traveling-wave tubes described in connection with the several figures of the drawings provides relatively constant ampliiication Vthroughout a broad band of frequencies; more specifically, these amplifiers may be constructed to provide substantially constant amplification throughout the U. H. F. television range of frequencies. The tubes do not require the bulky and heavy external magnetic structures utilized in prior art devices to form a magnetic collimating field for coniining the electron beam to its desired path; consequently, tubes constructed in accordance with the invention may be made relatively small and thus are well suited for mounting in a domestic television receiver or similar device. Moreover, because the electrostatic lens systems employed by the invention ciectively center the electron stream upon a center plane rather than merely collimating the electrons to constrain them to paths parallel to such a plane, they are inherently more effective in restricting dispersion of the beam attributable to thermal and `space charge effects. The invention therefore permits the use of substantially narrower beams and, consequently, a greater degree of coupling between the beam and the signal Wave traveling along the transmission lines. The openating voltages required are relatively low and do not unduly burden the power supply of a television receiver. The transmission line structures are relatively simple in form and may be readily constructed by known methods, so that the tubes are not unduly expensive if manufactured on a mass-production basis.

. While particular embodiments of the present invention have beenshown and described, it is apparent that changes andrmodiications may be made without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all such changes and modiications as f-all within the true spirit and scope of the invention.

I claim:

1. An electron-discharge device of the traveling-Wave type comprising: an electron gun for projecting a beam of electrons along a reference path; a plurality of electrodes consecutively disposed along a predetermined portion of said reference path and individually Vcomprising iirst and second juxtaposed mutually insulated conductive elements respectively on opposite sides of said reference path for inducing transverse undulations of said beam; additional electrodes disposed along said reference path intermediate successive pairs of said irst-rnentioned electrodes and adapted to be maintained at a diiferentunidirectional operating potential than that applied to said first-mentioned electrodes to constitute therewith a series of electrostatic lenses along Isaid predetermined portion of'said reference path for conning said transverse-undulating beam to the region of said reference path; inductance means interconnecting at least some of said electrodes to form a wave-transmission line; and means coupling said wave-transmission line to an input signal source for providing transverse electric signal fields between said juxtaposed conductive elements and for producing a signal wave which travels along said line and interacts with said transverse-undulating beam to provide substantial reinforcement of 'said traveling signal wave, said Wave-transmission line having a substantially uniform low wave-propagationvelocity in a direction parallel to said reference path and an electrical lengthV Moreover, as the electron stream enters 13 which is large relative to the eiective Wavelength of said traveling signal wave.

2. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a beam of electrons along a reference path; a plurality of electrodes consecutively disposed along a predetermined portion of said reference path and individually comprising first and second juxtaposed mutually insulated conductive elements respectively on opposite sides of said reference path for inducing transverse undulations of said beam; additional electrodes disposed along said reference path intermediate successive pairs of said rst-mentioned electrodes and adapted to be maintained at a different unidirectional operating potential than that applied to said rst-mentioned electrodes to constitute therewith a Iseries of electrostatic lenses along said predetermined portion of said reference path for conining said transverse-undulating beam to the region of said reference path; respective inductance means interconnecting at least some of said conductive elements on each side of said reference path to form a balanced pair of wave-transmission lines; and means coupling 'said wave-transmission lines in push-pull relation to an input signal source for providing transverse electric signal fields between said juxtaposed conductive elements and for producing a push-pull signal wave which travels along said lines and interacts with said transverse-undulating beam to provide substantial reinforcement of said traveling signal wave, said wave-transmission lines having substantially uniform low wave-propagation velocities in a direction parallel to said reference path and electrical lengths which are large relative to the eective wavelength of said traveling signal wave.

3. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a beam of electrons along a reference path; a beam-limiting electrode, included in said electron gun, for intercepting a major portion of said beam of electrons to form an electron beam of predetermined cross-sectional configuration; a. plurality of electrodes consecutively disposed along a predetermined portion of said reference path and individually comprising iirst and second juxtaposed mutually insulated conductive elements respectively on opposite sides of said reference path for inducing transverse undulations of said beam; additional electrodes disposed along said reference path intermediate successive pairs of said first-mentioned electrodes and adapted to be maintained at a different unidirectional operating potential than that applied to said first-mentioned electrodes to constitute therewith a series of electrostatic lens along said predetermined portion of said reference path for conning said transverse-undulating beam to the region of said reference path; nductance means interconnecting at least some of said electrodes to form a wave-transmission line; and means coupling said wave-transmission line to an input signal source for providing transverse electric signal elds between said juxtaposed conductive elements and for producing a signal wave which travels along said line and interacts with said transverse-undulating beam to provide substantial reinforcement of said traveling signal wave, said wave-transmission line having a substantially uniform low wave-propagation velocity in a direction parallel to said reference path and an electrical length which is large relative to the eiective wavelength of said traveling signal wave.

4. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a beam of electrons along a reference path; a plurality of electrodes consecutively disposed along a predetermined portion of said reference path and individually comprising rst and second juxtaposed mutually insulated conductive elements respectively on opposite sides of said reference path for inducing transverse undulations of said beam; additional electrodes disposed along said reference path intermediate successive pairs of said first-mentioned electrodes and adapted to be maintained at a different -unidi-- rectional operating potential than that applied to said rst-mentioned electrodes to constitute therewith a series of electrostatic lenses along said predetermined portion of said reference path for conning said transverse-um dulating beam to the region of said reference path; a rst helical conductive winding having preselected turns electrically connected to said first-mentioned electrodes; a second helical conductive winding, electrically coupled to said first winding for radio frequencies only, and having preselected turns electrically connected to said additional electrodes to form, in combination with said first winding and said first-mentioned electrodes, a wave-transmission line; and means coupling said helical windings to an input signal source for providing transverse electric signal iields between said juxtaposed conductive elements and for producing a signal wave which travels along said line and interacts with said transverse-undulating beam to provide substantial reinforcement of said traveling signal wave, said wave-transmission line having a substantially uniform low wave-propagation velocity in a direction parallel to said path and an electrical length which is large relative to the effective wavelength of said traveling signal wave.

5. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a beam of electrons along a reference path; la plurality of electrodes consecutively disposed along a predetermined portion of said reference path and individually comprising iirst and second juxtaposed mutually insulated conductive plates respectively on opposite sides of said reference path for inducing transverse undulations of said beam; a iirst pair of bifllar helical conductive windings electrically coupled to each other for radio frequencies only and individually having preselected turns electrically connected to alternate ones of the group comprising said iirst conductive plates to form therewith a first wavetransmission line; a second pair of bilar helical conductive windings electrically coupled to each other for radio frequencies only and individually having preselected turns electrically connected to alternate ones of the group comprising said second conductive plates to form therewith a second-wave-transmission line substantially identical with said first line; means fo-r maintaining said windings of each of said wave-transmission lines at substantially different unidirectional operating potentials, each substantially equal to the steady-state potential of the corresponding Winding of the other of said lines, for establishing a series of electrostatic lenses along said predetermined portion of said reference path to confine said transverse-undulating beam to the region of said reference path; and means coupling said pairs of bilar helical windings to an input signal source in push-pull relation, for providing transverse electric signal fields between said juxtaposed conductive plates 'and for producing a push-pull signal wave which travels along said lines and interacts with said transverse-undulating beam to provide substantial reinforcement of said traveling signal Wave, said wave transmission lines having substantially uniform low-wave-propagaticn velocities in a direction parallel to said path and electrical lengths which are large relative t-o the eiective wavelength of said traveling signal wave.

6. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a beam of electrons along a reference path; a plurality of electrodes consecutively disposed along a predetermined pc-rtion of said reference path and individually comprising first and second juxtaposed mutually insulated conductive elements respectively on opposite sides of said reference path for inducing transverse undulations of said beam; inductive means interconnecting said conductive plates to form a wave-transmission line; a plurality of interconnected apertured electrodes disposed along said reference path intermediate successive pairs of said first-mentioned electrodes; means for maintaining said aperturedV electrodes and said first-mentioned electrodes at different average unidirectional operating potentials tofestablish a series of electrostatic lenses along said predetermined portion of said reference path for confining said transverse-undulating beam to the region of said reference path; and means coupling said Wave-transmission line to an input signal source for providing transverse electric Signal fields between said juxtaposed conductive plates and for producing a signal wave which travels along said line and interacts with said transverse-undulating beam to provide substantial reinforcement of said traveling signal Wave, said wave-transmission line having a substantially uniform low Wave-propagation velocity in `a direction parallel to said path and an electrical length which is large relative to the effective Waveiength of said traveling signal wave.

7. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a beam of electrons along a reference path; a plurality of elec* trodes consecutively disposed along a predetermined portion of said reference path and individually comprising first and second juxtaposed mutually insulated conductive plates respectively on opposite sides of said reference path for inducing transverse undulations of said beam; a first group of inductance coils individually interconnecting adjacent pairs of the group comprising said rst conducting plates to form therewith a rst Wave-transmission line; a second group of inductance coils individually interconnecting adjacent pairs of the groupcomprising said second conducting plates to form therewithV a second Wave-transmission line substantially identical with said first line; a plurality of interconnectedr apertured electrodes disposed along said reference pathintermediate successive pairs of said first-mentionedk electrodes; means for maintaining said apertured electrodes and said firstmentioned electrodes at different average unidirectional operating potentials to establish a series of electrostatic lenses along said predetermined portion of said reference path for confining said transverse-undulating beam to the region of said reference path; and means coupling said wave-transmission lines in push-pull relation to an input signal source for providing transverse electric signal fields between said juxtaposed conductive plates and for prodncing a push-pull signal Wave which travels along said lines and interacts with said transverse-undulating beam to provide substantial reinforcement of said traveling signal Waves, said Wave-transmission lines having substantially uniform low Wave-propagation velocities in a direction parallel to said path and electrical lengths which are large relative to the effective wavelengths of said traveling signal Wave.

References Cited in the file of this patent UNTED STATES PATENTS Fritz Mar. 4, 1941` 

